Olefins such as butenes and pentenes are useful in preparing a wide variety of derivative end products. Examples of such end products include alcohols, aldehydes acids and esters. The butenes and pentenes can also be oligomerized to form higher olefins having eight or more carbons. The higher olefins may be linear or they may have one or more alkyl branches. The higher olefins can then be converted to alcohols, aldehydes, acids and esters.
Butenes used in preparing olefin derivative products are typically made by cracking hydrocarbon feedstocks, i.e., producing low molecular weight hydrocarbons from high molecular weight hydrocarbons. Cracking of hydrocarbon feedstocks can be accomplished catalytically or non-catalytically. Non-catalytic cracking processes are described, for example, in Hallee et al., U.S. Pat. No. 3,407,789; Woebcke, U.S. Pat. No. 3,820,955, DiNicolantonio, U.S. Pat. No. 4,499,055 and Gartside et al., U.S. Pat. No. 4,814,067. Catalytic cracking processes are described, for example, in Cormier, Jr. et al., U.S. Pat. No. 4,828,679; Rabo et al., U.S. Pat. No. 3,647,682; Rosinski et al., U.S. Pat. No. 3,758,403; Gartside et al., U.S. Pat. No. 4,814,067; Li et al., U.S. Pat. No. 4,980,053; and Yongqing et al., U.S. Pat. No. 5,326,465.
One problem with using a hydrocarbon cracking unit to produce olefins is that the olefins contain a significant degree of alkyl branched olefin. For example, in a butenes stream, isobutene must first be removed before the butenes are directed to an oligomerization unit. The presence of isobutene in the butenes feed will result in branched higher olefin, which leads to branched alcohols. Branched alcohols have relatively little commercial value because they result in inferior plasticizers.
Another problem with olefin produced by a hydrocarbon cracking unit is that the olefin contains significant quantities of sulfur and nitrogen compounds. These compounds deactivate the acidic catalysts used in olefin derivative processes, such as olefin oligomerization. For example, Bodart, U.S. Pat. No. 5,432,243, and Debras et al., U.S. Pat. No. 4,861,939, disclose that arsine and carbonyl sulfide (COS) can be problematic in the olefin derivative process unless the contaminants are removed by additional purification equipment. U.S. Pat. No. 5,146,042 to Gurak et al. suggests that sulfur contaminants in C2 to C4 olefin can lead to undesirable side reactions in higher olefin and olefin derivative processes. Purification of such olefin requires that the contaminants be extracted into selected hydrocarbons followed by the distillation of the cleaned, lighter olefin from the hydrocarbons. Alternatively, nickel catalysts can be used to remove the sulfur contaminants. The equipment required to remove sulfur from an olefin is generally quite large in scale and quite expensive to operate.
Additional separations, such as diene removal, iso-alkene removal, and/or paraffin removal may be required depending upon the hydrocarbon source used in the cracking unit. As an example, the butenes stream from a hydrocarbon cracking unit contains significant amounts of butadiene and isobutene that must be removed. In U.S. Pat. No. 6,049,017 to Vora et al., the butadiene is removed by a controlled hydrogenation process. The isobutene is removed catalytically by contacting the butenes stream with methanol in a methyl-t-butylether (MTBE) reactor. The isobutene is converted to MTBE and the normal butenes and butane pass through the MTBE reactor. The normal butenes and butanes are then directed to a butane cracking unit to produce ethylene and propylene or to an oligomerization unit.
Generally, in the production of higher olefin, butanes are not removed from the butenes stream because a once through or low recycle higher olefin process is used. Instead, butanes are separated from the higher olefin product, which is a much easier and less costly separation. For example, the olefin content of a butenes stream from a steam cracking unit is typically about 60% by weight. The butenes stream is directed to the higher olefin unit at a conversion per pass of about 50% to 70%. There is little or no recycle, and the butanes are easily separated from the higher olefin product. However, there are several disadvantages to this process. One, 30% to 50% of the olefin in the feed is not converted to the desired product resulting in overall low process yields. Two, the high conversion per pass process results in a lower selectivity to the more desirable alpha-olefins. Alpha-olefins are olefins that contain the carbon-carbon double bond between the first and second carbon.
A high recycle, low conversion per pass process may address both of these disadvantages, however, such a process requires the availability of an olefin stream with a high olefin content to maintain the olefin concentration in the feed at an acceptable level. A butenes stream from a cracking unit, has a low olefin content. Consequently, a significant portion of the paraffins must be removed from the butenes stream if a high recycle, low conversion per pass process is to be used. This removal process can be a difficult and expensive task because of the relatively close boiling ranges of the components.
Removing various chemical contaminants from an olefin stream for producing an olefin derivative product can be a technically difficult process depending upon the feed specifications for the process. Therefore, if one could minimize or avoid the paraffin and contaminant removal process by having available an olefin stream with low levels of paraffin, alkyl branching, diene, and/or contaminant levels the costs of removing these components would be minimized or eliminated altogether.